**3. Closed and semi-closed systems for Arabidopsis**

Some hydroponic systems are open systems that add new media and do not reuse or recycle old media [17]. Providing a continuous supply of defined nutrient or drug-containing solution makes open systems costly and does not take advantage of the ease with which hydroponic nutrients can be recycled or continuously re-used. In several of the above Kratky-type methods for Arabidopsis hydroponics, there is regular, but infrequent (weekly) modification of the nutrient solution and they provide a semi-closed, constant, uncycled medium. Closed and semi-closed systems

that use flow, such as the nutrient film technique (NFT), deep flow technique (DFT) and aeroponics (misted nutrient solution sprayed on the roots), cycle a larger volume of nutrient medium. However, both cycled and uncycled hydroponics share the problem that the ionic balance or salinity within the nutrient medium can change over time [18, 19]. Computational approaches based on continuous read-out from ion-selective electrodes provide real-time optimization of hydroponic media [20–22]. Such optimization in closed systems require, as part the enclosure, additives or scrubbers (such as ion exchange resins) to add or remove certain ions or nutrients.

Semi-closed and closed, aseptic systems for Arabidopsis growth are useful for (1) drug discovery, (2) gene discovery, (3) plant-microbe interactions, and (4) growth in non-terrestrial environments. The advantage of closed or semi-closed hydroponic systems for discovery of drugs effective against plants is that, unlike the situation in animals where target animals are not available for ethical or logistical reasons, it provides a method to evaluate the living target organism from early to late stages of growth [23]. If done on sufficient scale, it can be a form of high throughput in vivo screening. Whole organism screening covers important drug or herbicide properties such as uptake, efficacy, and breakdown. However, the frequent use of Arabidopsis, and other model organisms, e.g., duckweed (*Lemna* spp.) and the model grass, *Brachypodium distachyon*, as test species for drug discovery, particularly for herbicides, is problematic [24]. Model species may not have the same mechanisms for uptake, delivery, and metabolism of the drugs as agricultural weeds and crops. They have different life cycles and environmental preferences and constraints. Therefore, it is important to extend the work on closed and semi-closed systems using Arabidopsis to crop species and weed species, as described below.

This is also true for gene discovery. Identifying and preliminary mapping of multi-genic quantitative trait loci (QTLs) that produce desirable phenotypes is possible with recombinant inbred lines (RILs) and near isogenic lines (NILs) of Arabidopsis, if there is minimal environmental contribution to the character. The standard conditions of closed or semi-closed hydroponics seem ideally suited for these studies. When soil is used, it could be a source of irreproducibility. Growth conditions using soils produce irreproducible Arabidopsis leaf phenotypes in different labs, even controlling for many environmental variables and nutrient conditions [25]. Similar multi-lab reproducibility experiments with hydroponically grown Arabidopsis plants are not available, but provide an exciting opportunity for new study. Exploiting closed or semi-closed hydroponics tested with Arabidopsis could be an important step in speed breeding and the analysis of RILs or NILs of fast-cycling crop species [26], as described below.

Sequencing-based analysis of bacterial communities on plants reveals the diversity and complexity of the interaction of plants with the microflora of the rhizosphere [27]. Hydroponic approaches to analysis of Arabidopsis-microbe interactions provide a way to monitor how multiple bacterial species colonize the root or interact with each other to form these complex interactions [28]. These approaches require aseptic culture of Arabidopsis and controlled introduction of monocultures of bacteria into the media. Harris et al. [28] adapted the simple, inexpensive, closed hydroponic system described above [5] to analyze the colonization of Arabidopsis roots with *Pseudomonas*, *Arthrobacter*, *Curtobacterium*, and *Microbacterium* species.

Completely enclosed, but not necessarily hydroponic, systems were common for early studies on the growth of plants in extraterrestrial environments (see review, [29]). The cultivation system of choice is recirculating, enclosed hydroponics, however, for future space flights that would include plants in a life support system [30]. The new recirculating enclosed system by NASA and ORBITEC, however, has Arcelite as a root support matrix with porous tubes to pump the nutrient solution [31].

**19**

*Hydroponic Systems for Arabidopsis Extended to Crop Plants*

The initial tests of the Plant Habitat-01 in the International Space Station will be studies on Arabidopsis. Future experiments will include durum wheat, as shown in **Figure 3**. Studies on soil-less bioregenerative life support systems funded by the European Space Agency use the nutrient film technique (NFT) of hydroponics [32], with the caveat that the implementation of such a technique in microgravity is yet to come. All of these life support systems are gas-tight enclosures that will monitor gases emitted by the plants, because the recycling of carbon dioxide and oxygen by plants or other photosynthetic organisms will be a necessity in long-term flights

*Advanced plant habitat of NASA showing growing wheat plants in an enclosed chamber with an Arcelite substrate (https://www.nasa.gov/sites/de-fault/files/atoms/files/ad-vanced-plant-habitat.pdf).*

**4. Extending closed and semi-closed systems of Arabidopsis to crop** 

A semi-closed hydroponic system that is very close to those described above for Arabidopsis is the single-tube hydroponics of Kuroda and Ikenaga [33]. As in the procedures for Arabidopsis used by Nguyen et al. [6] and Robison et al. [3], the plants initially germinate on an agar (gelrite) medium containing ½ strength Murashige and Skoog medium. They grow a variety of crop plants, i.e., rice, soybean, Azuki beans, and corn, instead of Arabidopsis. At 2 weeks, Kuroda and Ikenaga [33] transplant the intact germinated seedlings into 12 ml polypropylene tubes with two holes cut into the sides to allow the entry and exit of hydroponic medium (1/10 strength Murashige and Skoog medium). A covered outer tray contains the medium and a rack for the tubes in which the plants grow. The tube supports the plant during culture and contains the root ball of each plant, thereby facilitating removal for analysis without damaging the roots. The size of the culture tube is larger than that used by Tocquin et al. [2] for Arabidopsis because the seeds and new roots of crop plants are much larger, but the tray system for growth in hydroponics is very similar. For soybean, an additional prop supports the plant during growth. Rice and soybean plants grown in single tube hydroponics produce high viability seed with seed weights equal to or exceeding plants grown in the field. Single tube hydroponics facilitate analyzing and screening the T1 seeds from the transgenic plants with shorter generation times and small amounts of seeds. The hydroponic system of Conn et al. [4] directly translates an Arabidopsis hydroponic culture system to crop plants. They use a system of hole-punched plastic trays to start the plant, followed by transplantation of the seedlings into a plastic tube that can be set in a larger tray with aeration. The 50 ml tube in this case, although it did not confine the root as much those used by Kuroda and Ikenaga [33],

*DOI: http://dx.doi.org/10.5772/intechopen.89110*

such as those to Mars.

**species**

**Figure 3.**

*Hydroponic Systems for Arabidopsis Extended to Crop Plants DOI: http://dx.doi.org/10.5772/intechopen.89110*

**Figure 3.**

*Urban Horticulture - Necessity of the Future*

fast-cycling crop species [26], as described below.

Sequencing-based analysis of bacterial communities on plants reveals the diversity and complexity of the interaction of plants with the microflora of the rhizosphere [27]. Hydroponic approaches to analysis of Arabidopsis-microbe interactions provide a way to monitor how multiple bacterial species colonize the root or interact with each other to form these complex interactions [28]. These approaches require aseptic culture of Arabidopsis and controlled introduction of monocultures of bacteria into the media. Harris et al. [28] adapted the simple, inexpensive, closed hydroponic system described above [5] to analyze the colonization of Arabidopsis roots with *Pseudomonas*, *Arthrobacter*, *Curtobacterium*, and *Microbacterium* species. Completely enclosed, but not necessarily hydroponic, systems were common for early studies on the growth of plants in extraterrestrial environments (see review, [29]). The cultivation system of choice is recirculating, enclosed hydroponics, however, for future space flights that would include plants in a life support system [30]. The new recirculating enclosed system by NASA and ORBITEC, however, has Arcelite

as a root support matrix with porous tubes to pump the nutrient solution [31].

nutrients.

that use flow, such as the nutrient film technique (NFT), deep flow technique (DFT) and aeroponics (misted nutrient solution sprayed on the roots), cycle a larger volume of nutrient medium. However, both cycled and uncycled hydroponics share the problem that the ionic balance or salinity within the nutrient medium can change over time [18, 19]. Computational approaches based on continuous read-out from ion-selective electrodes provide real-time optimization of hydroponic media [20–22]. Such optimization in closed systems require, as part the enclosure, additives or scrubbers (such as ion exchange resins) to add or remove certain ions or

Semi-closed and closed, aseptic systems for Arabidopsis growth are useful for (1) drug discovery, (2) gene discovery, (3) plant-microbe interactions, and (4) growth in non-terrestrial environments. The advantage of closed or semi-closed hydroponic systems for discovery of drugs effective against plants is that, unlike the situation in animals where target animals are not available for ethical or logistical reasons, it provides a method to evaluate the living target organism from early to late stages of growth [23]. If done on sufficient scale, it can be a form of high throughput in vivo screening. Whole organism screening covers important drug or herbicide properties such as uptake, efficacy, and breakdown. However, the frequent use of Arabidopsis, and other model organisms, e.g., duckweed (*Lemna* spp.) and the model grass, *Brachypodium distachyon*, as test species for drug discovery, particularly for herbicides, is problematic [24]. Model species may not have the same mechanisms for uptake, delivery, and metabolism of the drugs as agricultural weeds and crops. They have different life cycles and environmental preferences and constraints. Therefore, it is important to extend the work on closed and semi-closed systems using Arabidopsis to crop species and weed species, as described below. This is also true for gene discovery. Identifying and preliminary mapping of multi-genic quantitative trait loci (QTLs) that produce desirable phenotypes is possible with recombinant inbred lines (RILs) and near isogenic lines (NILs) of Arabidopsis, if there is minimal environmental contribution to the character. The standard conditions of closed or semi-closed hydroponics seem ideally suited for these studies. When soil is used, it could be a source of irreproducibility. Growth conditions using soils produce irreproducible Arabidopsis leaf phenotypes in different labs, even controlling for many environmental variables and nutrient conditions [25]. Similar multi-lab reproducibility experiments with hydroponically grown Arabidopsis plants are not available, but provide an exciting opportunity for new study. Exploiting closed or semi-closed hydroponics tested with Arabidopsis could be an important step in speed breeding and the analysis of RILs or NILs of

**18**

*Advanced plant habitat of NASA showing growing wheat plants in an enclosed chamber with an Arcelite substrate (https://www.nasa.gov/sites/de-fault/files/atoms/files/ad-vanced-plant-habitat.pdf).*

The initial tests of the Plant Habitat-01 in the International Space Station will be studies on Arabidopsis. Future experiments will include durum wheat, as shown in **Figure 3**. Studies on soil-less bioregenerative life support systems funded by the European Space Agency use the nutrient film technique (NFT) of hydroponics [32], with the caveat that the implementation of such a technique in microgravity is yet to come. All of these life support systems are gas-tight enclosures that will monitor gases emitted by the plants, because the recycling of carbon dioxide and oxygen by plants or other photosynthetic organisms will be a necessity in long-term flights such as those to Mars.
